The jellyfish Rhizostoma pulmo is one of the largest and most distributed scyphozoan species in the Mediterranean Sea, characterized by seasonal and inter-annual fluctuations in population size, with alternance of high abundance and rarity periods. In spite of a substantial number of studies on R. pulmo biology and ecology, the diversity and abundance of the jellyfish-associated microbiome is largely unknown. In the present study, we investigated the abundance and metabolic diversity of bacteria associated with three fractions of the R. pulmo jellyfish, namely two distinct body fractions, i.e. the umbrella and the oral arms, and the mucus secretion. Different bacterial metabolic pathways have been identified among the above mentioned fractions, with the highest value of abundance and metabolic activity in the mucus compared to the umbrella and oral arms. These findings are discussed in the framework of the ecology of the species.
